The present invention relates to a catalyst composition and a process for the oligomerization, especially tri- and tetramerization, of ethylene.
Existing processes for the production of linear alpha olefins (LAOs), including comonomer-grade 1-hexene and 1-octene, rely on the oligomerization of ethylene. These processes have in common that they lead to a product distribution of ethylene-oligomers of chain length 4, 6, 8 and so on. This is due to a chemical mechanism which is widely governed by competing chain growth- and displacement reaction steps, leading to a Schulz-Flory- or Poisson-product distribution.
From the marketing point of view, this product distribution poses a formidable challenge for the full-range alpha olefins producer. The reason is that each market segment served exhibits a very different behavior in terms of market size and growth, geography, fragmentation etc. It is, therefore, very difficult for the producer to adapt to the market requirements since part of the product spectrum might be in high demand in a given economic context, while at the same time other product cuts might not be marketable at all or only in a marginal niche. Currently, the highest-value LAO product is comonomer-grade 1-hexene for the polymer industry, while 1-octene demand is also growing at a considerable rate.
WO 2009/006979 A2 describes a catalyst composition and a process for the di-, tri- and/or tetramerization of ethylene. The catalyst composition comprises a chromium compound, a ligand of, for example, the general structure R1R2P—N(R3)—P(R4)—N(R5)—H and a co-catalyst acting as an activator. The ligand's substituents R1, R2, R3, R4 and R5 are independently a number of functional groups, comprising (among others) C1-C10-alkyl, aryl and substituted aryl.
The chromium source is CrCl3(THF)3, Cr(III)acetylacetonate, Cr(III)octanoate, Cr-hexacarbonyl, Cr(III)-2-ethylhexanoate and (benzene)tricarbonyl-chromium (THF=tetrahydrofuran).
The co-catalyst or activator is trimethylaluminum, triethylaluminum, triisopropylaluminum, triisobutylaluminum, ethylaluminumsesquichloride, diethylaluminum chloride, ethylaluminumdichloride, methylaluminoxane or a combination comprising at least one of the foregoing.
This prior art discloses a class of catalyst systems for selective ethylene oligomerization reactions.
For example, one embodiment uses a specific catalyst composition, chosen from this class of catalyst systems, for the highly selective trimerization of ethylene to afford high yields of 1-hexene.
This choice of catalyst constituents comprises CrCl3(THF)3 as chromium source, triethylaluminum as activator, and (Ph)2P—N(i-Pr)—P(Ph)-N(i-Pr)—H as ligand for the catalytically active complex (Ph=phenyl group, i-Pr=isopropyl group). This ligand features the typical PNPN—H-backbone, which is why this class of compounds, regardless of the precise nature of its substituents, is often referred to as a “PNPN—H-ligand”.
WO 2010/115520 A1 describes essentially modified catalyst systems of the general type already disclosed in WO2009/006979 A2. These modified systems take advantage from the same PNPN—H-type ligands that were already known. However, now a “modifier” is added to the system, (but not limited to) ammonium or phosphonium salts of the type [H4E]X, [H3ER]X, [H2ER2]X, [HER3]X or [ER4]X (with E=N or P, X═Cl, Br or I and R=alkyl, cycloalkyl, acyl, aryl, alkenyl, alkynyl etc.).
Preferred embodiments involve, for instance, modifiers such as tetraphenylphosphonium chloride, tetraethylammonium chloride-monohydrate, triethylamine-hydrochloride etc. Also, as a “type [ER4]X”-modifier, dodecyl trimethylammonium chloride can advantageously be used, due to its low price, abundant supply and good solubility in the reaction solution. By means of the halogen-containing modifier, the catalyst system allows for an independent adjustment of the [Cr]/[Halogen] molar ratio in the resulting catalytically active species which is formed in-situ under oligomerization conditions.
On technical scale, the prior art oligomerization technologies described above are mainly suitable for the production of 1-butene and 1-hexene for use as co-monomers in the polyethylene (PE) production, especially for linear low density polyethylene (LLDPE).
Currently, most of the co-monomer used for PE production is 1-butene followed by an increasing 1-hexene demand. However, some high-quality PE-materials featuring high tensile strength and crack resistance require 1-octene as co-monomer. So far, the largest quantity of 1-octene is obtained from full-range LAO-processes or extraction from Fischer-Tropsch streams. Since these technologies are burdened with rather big amounts of other products than 1-octene, their economic viability varies greatly with technological and economic boundary conditions. This pertains to infrastructure, market access and price development for the full-range products under the local boundary conditions.
It is, therefore, desirable to have catalyst systems and processes with a higher selectivity towards 1-octene available. Since 1-hexene is also a valuable co-monomer, combined 1-hexene/1-octene processes are economically interesting as well.